The present invention relates to an electronic camera and, more particularly, to an electronic camera capable of preventing deterioration of the picture quality and deterioration of the quality of a finder image caused by adhesion of dust, and at the same time allowing easy replacement/disassembly of optical members.
In an electronic camera, if dust adheres to an optical-member through which incident light to an image sensing element passes, a photographed picture is influenced by the dust. When this is the case, it is highly likely that even rubbish of a very small size, i.e., dust, which is not a problem in a silver halide camera, appears as a black dot on a photographed frame and deteriorates the picture quality. This is mainly because the size of the image sensing surface of an image sensing element is smaller than that of a silver halide film used in a silver halide camera. Adhesion of dust to optical members is particularly a big problem for an image sensing element having a high pixel density (multiple pixels), because such an image sensing element is required to have a high picture quality.
In addition, the zoom magnification at the focusing plane of an optical finder of an electronic camera is larger than that of a silver halide camera. Therefore, even rubbish of a very small size, i.e., dust, which is of no problem in a silver halide camera is highly likely to appear as a black dot in a finder field and deteriorate the quality of a finder image.
To solve these problems, in a digital still camera disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-109203, a lens and an optical filter are fixed in a sealed state in a cylindrical sealing member, and this cylindrical sealing member is attached to a substrate for mounting an image sensing element so as to cover this image sensing element tightly. That is, in this camera, a closed space is formed between the lens and the optical filter through which incident light to the image sensing element passes, and around the image sensing element. This prevents deterioration of the picture quality caused by adhesion of dust.
In this digital still camera disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-109203, however, once a unit including the components in the cylindrical sealing member is assembled, it is not easy to disassemble the unit in order to replace the parts in the cylindrical sealing member. That is, this conventional structure in which the optical members in the vicinity of the image sensing element are covered with the sealing member to thereby prevent invasion and adhesion of dust still has antinomic problems: improving the picture quality and facilitating replacement/disassembly of optical members.
Also, a camera containing a focal-plane shutter having front and rear curtains is known as a silver halide camera (a camera using a silver halide film). Since the film surface must be shielded except during exposure in a silver halide camera, this focal-plane shutter operates as follows.
Before exposure, the front curtain is always in a shielding position, and the rear curtain is in a retraction position. By a release operation, the front curtain moves to a retraction position to uncover the film surface, and the film surface is exposed to light. After a predetermined time has elapsed, the rear curtain in the retraction position moves to shield the film surface and complete the exposure. That is, the operation timings of the front and rear curtains determine the shutter speed. When the shutter speed is low (e.g., 1/500 SEC or more), the rear curtain moves after the front curtain retracts. When the shutter speed is high, the rear curtain begins to move while the front curtain is moving. In this case, the exposure time is determined by the width (size) of a slit gap between the trailing end of the front curtain and the leading end of the rear curtain (slit shutter mode).
In an electronic camera, on the other hand, a time corresponding to the exposure time in a silver halide camera can be set by an element shutter which is defined by turn on/off of an image sensing element for photoelectric conversion, e.g., a CCD. However, even an electronic camera uses a mechanical shutter for the following reasons. First, an interlace CCD requires the operation of a mechanical shutter at the end of exposure. Second, a progressive CCD must be shielded by a mechanical shutter immediately after exposure is completed by an element shutter, in order to prevent smear (therefore, exposure is preferably terminated by a mechanical shutter).
For example, Jpn. Pat. Appln. KOKAI Publication No. 11-122542 disclosed an electronic camera containing a focal-plane shutter having only one of front and rear curtains. Also, Jpn. Pat. Appln. KOKAI Publication No. 11-218838 disclosed an electronic camera containing a focal-plane shutter having a large number of sectors functioning as a shutter which also serves as a stop. The latter publication also describes that a focal-plane shutter having front and rear curtains is also usable.
In an electronic camera containing the conventional focal-plane shutter, e.g., in each of the electronic cameras disclosed in the above two publications, the shutter is placed closely in front of a CCD. This is so because the role of the focal-plane shutter is to shield the image sensing surface of the CCD. This similarly holds for a silver halide camera.
The focal-plane shutter operates a considerable number of times at high speed. Therefore, wear caused by contact of the sectors produces wear dust. In a silver halide camera, deterioration of the picture quality caused by this wear dust is hardly a problem because the size of a silver halide film is large. However, as described previously, if wear dust adheres to the cover glass of a CCD in an electronic camera, this adhered wear dust appears as a black dot in a photographed frame and deteriorates the picture quality. The reasons are, for example, that the image sensing surface of a CCD is much smaller than a silver halide film, and the cover glass is very close to the image sensing surface.